1. Field of the Invention
The present invention relates to a focus detection device for a camera which corrects the focal point position of an interchangeable lens in accordance with the characteristics of the interchangeable lens and the characteristics of the focus detection device.
2. Description of the Related Art
Cameras which have a focal point position correction function for interchangeable lenses are known. For example, FIG. 11 shows a prior art type of focus detection device 12 which is used in a camera with an interchangeable lens. The focus detection device 12 is known as a positional difference type focus detection device because a focal point is detected based upon the difference in the imaging positions of two partial beams. In operation, two partial beams are derived from an exit beam of an interchangeable lens 14. In order to derive the two partial beams, the exit beam of the interchangeable lens 14 is focused on a converter lens 18 which directs the exit beam to a stop mask 16 having two detection apertures 16a, 16b. The size of a detection aperture is shown by the dotted line 24 in FIG. 12, and two entry pupils 16a', 16b ' are also shown. The beams directed through the apertures 16a, 16b are focussed by re-imaging lenses 20a and 20b onto minute one-dimensionally arrayed photoelectric elements (CCDs, etc.) 22a and 22 b, respectively. The one-dimensionally arrayed photoelectric elements 22a, 22b detect the distribution of light intensity in their array direction, and in a direction perpendicular to the array in order to determine a focal point. In general, the direction of the arrays 22a, 22b and the direction of the mask apertures 16a, 16b are in agreement. The directions of the mask apertures 16a, 16b and the directions perpendicular to the mask apertures 16a, 16b are known as the focus directions.
In these types of cameras, an F value (which will also be referred to below as the aperture equivalent F value) refers to the size of a detection aperture in a focus detection element of the device. When a lens is substituted, and the F value of the substituted lens is equal to the F value of the focus detection element, focal point position correction is performed without problems. However, for example, when the F value of the focus detection element is set to 4, and the substituted lens has an F value of 2, at the moment of photography the resulting focal point position obtained from the focus detection element is displaced from the ideal focal point position.
In order to correct for the displacement in focal point position, prior art cameras have stored a correction value in a memory in the lens. For example, Japanese Laid-Open Patent Application 62-227108 ("JP-A-62-227108") discloses a camera wherein an F value correction quantity is stored in an interchangeable lens, and during focus detection by a focus detection device, the F value correction quantity is read out and the focal point position is corrected using the correction quantity.
However, when the F value correction quantity stored in the interchangeable lens corresponds to an aperture equivalent F value which differs from the aperture equivalent F value of the focus detection element, focal point correction will not be performed successfully. By way of example, when a camera has a focus detection element having an aperture equivalent F value of 8, and an interchangeable lens mounted on the camera has an F value correction quantity (the open aperture F4 difference) stored in the interchangeable lens corresponding to a focus detection element having an aperture equivalent F value of 4 (for example, a 50 turn/1.4 standard lens), focal point correction will not be performed successfully even though focus detection is performed.
In order to overcome the above-noted problem, in JP-A-62-227108, two or more F value correction quantities, for example, open aperture F4 difference and open aperture F8 difference, are stored in the interchangeable lens. When the interchangeable lens is mounted on a camera body having a focus detection element whose aperture equivalent F value is not 4 or 8, for example, a focus detection element having an F value of 5.6, an approximate correction value is calculated from the stored aperture F4 difference and aperture F8 difference. The approximate correction value may be calculated according to the formula: EQU 0.5.multidot.(open aperture F4 difference+open aperture F8 difference)=approximate correction value.
By using the calculated approximate correction value as the correction quantity, focal point correction becomes possible for optional combinations of interchangeable lenses and cameras.
However, in the prior art camera disclosed in JP-A-62-227108, the focus detection region is restricted to the vicinity of the photographic optical axis, and appropriate focal point correction becomes impossible away from the optical axis. Techniques are known for performing focal point correction with a focus detection element, wherein a region away from the photographic optical axis of a photographic field is used as a focal detection region. An example of such a technique is disclosed in Japanese Laid-Open Patent Publication 62-189415 ("JP-A-62-189415").
According to JP-A-62-189415, the focal point correction quantity for focus detection elements having different focus detection positions and detection directions is stored in an interchangeable lens. In general, even in the same kind of interchangeable lens, the focal point correction quantity will differ in accordance with the position and direction of detection in a photographic field of a focus detection region for the focus detection element. For example, FIG. 3 illustrates detection regions a, b, c, and d shown by rectangles in a photographic field 10. The directions of the long axes of the rectangles are the detection directions of the rectangles, and the centers of the rectangles are the detection positions. If the detection regions of an interchangeable lens (for example, a 50 mm/1.4 standard lens) are a, b, c, and d, the respective focal point correction quantities ha, hb, he, and hd relative to the focus detection element are previously stored in the interchangeable lens. When the detection region is b, and focus detection is performed, the focus detection element performs focal point correction by selectively reading hb from the interchangeable lens mounted on the camera body.
However, when the interchangeable lens is mounted on a camera body having a focus detection element with a detection region e which is different from regions a, b, c and d, as shown in FIG. 3, focal point correction can not be performed because the focal point correction quantity he relative to the detection region e is not stored in the interchangeable lens. Furthermore, although the correction quantity he may be stored in the interchangeable lens, since camera bodies normally differ and focus detection elements also differ, the quantity of storage required in the interchangeable lens increases.
Additionally, when a new camera body is developed, design limitations arise because the detection region of the focus detection element has to be loaded with the focal point correction quantity stored in the interchangeable lens. Even in the same kinds of interchangeable lenses, because the size of the detection aperture of the focus detection element will differ according to the detection position and detection direction, the amount of data relating to the focal point correction quantity which has to be stored in the interchangeable lens is still further increased.